1. Field of the Invention
The present invention relates to an optical pick-up device of a digital versatile disc player, and more particularly to an actuator which operates an optical pick up device.
2. Description of the Prior Art
An optical pick-up device is used in an optical disc player in order to record and reproduce information on and from an optical disc by using a laser beam. In general, an optical pick-up device includes a light source, a beam splitter which reflect a laser beam radiated from the light source toward an object lens, an object lens which focuses the laser beam on an optical disc surface, and a receiving device which receives the laser beam reflected on the optical disc surface. The optical disc device reads the information from the optical disc when it moves in the radial direction of the optical disc.
The information is recorded on tracks which are formed concentrically or spirally on the optical disc, and includes a plurality of pits. The sizes of the pits and the intervals of the tracks or pitches of optical discs are different from one another, depending on the types of the optical discs. In order to converge the laser beam on the pits, the optical pick-up device should move between the tracks accurately. The conversion of the laser beam on the pits is called "focusing", and the movement of the optical pick-up between the tracks is called "tracking". The focussing and the tracking of the optical pink-up device is accomplished by an actuator.
The optical pick-up device detects servo error signals comprising focusing error signals and tracking error signals by using laser beams reflected on the tracks, and the actuator drives the object lens in the focusing and tracking directions based on the servo error signals detected by the optical pick-up device. For the focusing of the optical pick-up device and for the tracking of the optical pick-up device in the radial direction of the optical disc, the actuator moves the optical pick-up device vertically with respect to the optical pick-up device.
Generally, a compact disc (hereinafter, referred to as a CD) is used as an optical disc for audio systems of a computer or a multimedia or for listening to music. The CD has advantages in that fewer errors are generated in the CD than in a floppy disc and in that the CD is more solid than the floppy disc, so is currently widely used as an information storing media currently. However, the amount of the information recorded on the CD is limited, and it is not suitable for moving pictures such as movies. A digital versatile disc (hereinafter, referred to as a DVD) has been developed in order to settle the above-mentioned problems of the CD. In the DVD, more information can be recorded than in the CD, the sizes of the pits on which information are recorded is smaller than those of the CD, and the pitches defined by the pits of the DVD are shorter than those of the CD.
Therefore, it is impossible to read information from a DVD by a reproducing device for CD. A reproducing device for DVD includes an optical pick-up device which can read information of both a CD and a DVD, the focussing and the tracking of the optical pick-up device being accomplished by an actuator.
FIG. 1 is an exploded perspective view for showing a conventional actuator for a DVD player. As shown in FIG. 1, the conventional actuator for a DVD player comprises a lens holder 14 which includes first and second object lens 11 and 12 for reading information from a CD or a DVD, focusing and tracking coils 15 and 16 which respectively accomplishes the focusing and the tracking functions of the first and second object lenses 11 and 12, and a yoke plate 18 which is connected to the lens holder 14 by a shaft 17.
The focusing and tracking coils 15 and 16 are mounted on the outer surface of the lens holder 14, and focusing and tracking magnets 19 which are respectively in magnetic conjunction with the focusing and tracking coils 15 and 16 are fixed to the yoke plate 18. The first and second object lenses 11 and 12 are focused by the focusing coil 15 and the focusing magnet 19 and then tracked by the tracking coil 16 and the tracking magnet 20, and the information is read by the CD or the DVD.
The shaft 17 which connects the lens holder 14 to the yoke plate 18 is inserted into a sleeve 13 which is formed on the lens holder 14. When the focusing coil 15 and the focusing magnet 19 are in magnetic conjunction with each other, the lens holder 14 moves along the length of the shaft 17 in order to focus the first and second object lenses 11 and 12, and the inner surface or the sleeve 13 then frictionally contacts with the shaft 17. The diameter of the sleeve 13 is enlarged by the frictional contact between the inner surface of the sleeve 13 and the shaft 17. Therefore, in order to prevent the enlargement of the diameter of the sleeve 13, the inner surface of the sleeve 13 and the outer surface of the shaft 17 are coated with a material whose frictional coefficient is small.
However, the coated layers of the sleeve 13 and the shaft 17 are gradually removed by the repetition of the focusing of the object lenses 11 and 12. As a result, since the friction force between the shaft 17 and the sleeve 13 is increased over time, thereby enlarging the diameter of the sleeve 13, the object lenses 11 and 12 does not accurately accomplish the focusing function thereof. Further, if external impacts are applied to the actuator 10, errors are generated when the object lenses 11 and 12 accomplish their focusing and tracking functions.
FIG. 2 shows a conventional actuator 30 which is disclosed in U.S. Pat. No. 4,866,690 issued to Takuya Tamaru et al. on Sep. 12, 1989 and entitled "Vibration Imparted Focus Search Device in an Optical Type Disc Playback Device".
The actuator 30 of Tamaru comprises a cylindrical lens holder 32 in which a sleeve 33 is formed, focusing and tracking coils 34 and 36 which are mounted on the outer surface of the lens holder 32, and a yoke plate which is inserted into the sleeve 33 and includes a shaft 58 for supporting the lens holder 32. On the outer surface of the lens holder 32 is connected a damper 38 which has a damper sleeve 40.
A plurality of yokes 52 are formed on the upper surface of the yoke plate 50, and between the opposing two yokes is fixed a plurality of magnets 54 which operate in magnetic conjunction with the focusing and tracking coils 34 and 36. On the upper surface of the yoke plate 50 is formed a damper shaft 56 which is inserted into the damper sleeve 40. The lens holder 32 is supported by the shaft 58 and the damper shaft 56.
According to the actuator 30 of Tamaru the shaft 58 is inserted into the damper sleeve 40 which is formed along the rotating axis of the lens holder 32, and rotatably and slidably supports the lens holder 32 with respect to the yoke plate 50.
The reference position of the lens holder 32 is determined in such a manner that the side surface of the tracking coil 36 is separated from yokes which are opposed to the magnets 54.
When a focusing servo (not shown) is switched ON, focusing control is accomplished, and when the focusing servo is switched OFF, the lens holder 32 returns to the reference position thereof by the damper 38 and slowly moves along the axis thereof. Then, a tracking servo (not shown) is switched OFF, and an AC voltage from an oscillator is applied to the tracking coil 36, and vibration is thus transferred to the lens holder 32 in the rotating direction of the lens holder 32 with respect to the rotating axis. The vibration enables the lens holder 32 to slide, thereby preventing stick slips between the shaft 33 and the sleeve.
However, the actuator 30 of Tamaru needs a separate vibrating imparting device for smooth sliding of the lens holder 32. Further, the actuator 30 has a disadvantage in that it cannot absorb external impacts.